1. Field of the Invention
The present invention relates to an apparatus for controlling the density of toner. More particularly, the present invention relates to a toner density control apparatus for use in a copying machine or the like of the type in which development with toner is carried out while automatically supplying the toner according to the measured difference between the reference density of the developer and the now existing density thereof.
2. Description of the Prior Art
An apparatus for control of toner density is known and used in automatic toner supply in accordance with the difference between the reference density of the developer and the now existing density thereof. Such a toner density control apparatus has been used in particular for a two-component development type of copying machines. The principle of the known toner density control apparatus will be described with reference to FIGS. 1A through 1C.
In FIG. 1A, 1 is a reference density plate, 2 is a light source and 3 is a photo cell. To detect the density of the reference density plate 1, for example, at the time of non-development, the light from the light source 2 is projected on the plate 1 and the reflected light from the plate is received by the photo cell 3. In a determined time (for example, during the development) the reference density plate 1 is rotated about its rotation axis 2' and the light from the light source 2 is made incident upon developer 5 through a glass plate 4. The photo cell 3 receives the reflected light from the developer to measure the density of the developer. The detection voltage of the density of the reference plate 1 is applied to the negative input of a comparator 6 through a terminal a.sub.1 of switch SWA shown in FIG. 1B. At the same time, the detection voltage charges a condenser C1 through R1 (at that time switch SWB is opened). With this detection voltage the condenser C1 is charged up to a level indicated by 7 in FIG. 1C.
On the other hand, the detection voltage of the density of developer 5 photo-electrically transduced by the photo cell 3 is applied to the positive input of the comparator 6 because switch SWA is in contact with terminal a.sub.2 and switch SWB is ON at the time of density detection of the developer 5. The density detection voltage obtained this time is represented by level 8 in FIG. 1C. By turn-ON of switch SWB the condenser C1 is discharged through R2 as suggested by the broken line in FIG. 1C. The output of comparator 6 continues to be "1" until the voltage of the condenser C1 drops down under the detection voltage level of the developer density. The output "1" from the comparator 6 is gated by a signel S1 which is "1" during the development thereby allowing the toner supply only during the time of development. Thus, toner is automatically supplied for a supply time .DELTA.T which is determined by: EQU .DELTA.T=k.multidot..DELTA.C
wherein,
C is the difference between the above two voltage levels; and PA0 k is a proportional constant determined by the function of discharge time constant .tau., k=f(.tau.) wherein the discharge time constant .tau.=C.sub.1 R.sub.2.
As will be understood from the above, when the values of C.sub.1 and R.sub.2 are once determined, the proportional constant k is directly and is uniformly determined by it. This brings about the following problems:
The consumption of toner per copy is not constant but variable depending on the kind of the original to be copied. Therefore, when a large number of copies are continuously made with a higher toner consumption per sheet, the toner supply cannot keep up with the toner consumption and gets behind the latter because of the time required for stirring toner, etc. In this case, the density control will reach an equilibrium prematurely at a lower density level than the reference level. On the contrary, when a number of copies are made from such originals which contain a large blank area and therefore consume a small amount of toner per sheet, the density control will reach an equilibrium at a higher density level than the reference level.
In this manner, according to the conventional density control system, the quality of development with toner varies greatly according to the kind of original to be copied and/or the number of copies to be continuously made. This causes the problem of degraded copies.